Plastic semiconductor packages are fabricated using lead frames. A conventional lead frame comprises a stamped, or etched metal strip, adapted to support multiple dice for packaging. A conventional plastic package includes a semiconductor die attached to a portion of the lead frame, and encapsulated in a plastic body. The lead frame supports the dice during the molding process, and provides the terminal leads in the completed packages.
One type of lead frame includes separate metal plates that function as power and ground buses for the die. Such a lead frame is described in U.S. Pat. No. 4,835,120 to Mallik et al.
FIG. 1 illustrates the lead frame 10, and FIG. 2 illustrates a package 11 constructed using the lead frame 10. As shown in FIG. 1, the lead frame 10 includes lead fingers 12 arranged in a peripheral pattern around a rectangular shaped opening 14. In the completed package 11, the lead fingers 12 are shaped to form package leads 30 (FIG. 2). The lead frame 10 also includes a power plate 16, and a ground plate 18, which are attached to the lead fingers 12 using adhesive layers 20, 22.
The power plate 16 covers one side of the opening 14, and provides a surface for mounting a semiconductor die 24 (FIG. 2). In the completed package 11, wires 26 are wire bonded to selected bond pads on the die 24 (e.g., Vcc pads) and to bonding sites on the power plate 16. Selected tabs 28 on the lead frame 10 provide conductive paths from the power plate 16 to appropriate power leads 30 for the package 11. In a similar manner, wires 32 are bonded to selected bond pads on the die 24 (e.g., Vss pads) and to bonding sites on the ground plate 18. Other selected tabs 28 on the lead frame 10 provide conductive paths from the ground plate 18 to appropriate ground leads 30 for the package 11. In addition to wires 26 and 32, wires 34 are bonded to selected bond pads on the die 24 (e.g., signal pads) and to appropriate signal leads 30 for the package 11. Following wire bonding, a transfer molding process can be used to form a plastic body 36 which encapsulates the wires 26, 32, 34, die 24, and lead fingers 12.
A positive aspect of the package 11 is that inductance between the power and ground pads on the die 24 is reduced. This results from multiple power and ground leads being replaced by a single power plate 16 and ground plate 18, which are low inductance metal planes. The power plate 16 and the ground plate 18 also function as heat sinks for dissipating heat generated by the die 24.
A negative aspect of the package 11 is that the power plate 16 and the ground plate 18 are both located on the same side of the lead frame 10. This construction can stress the lead frame 10 and cause the plastic body 36 to bow. Also, heat transfer is only provided from a backside of the die.
The present invention is directed to a multi layered lead frame that includes ground and power plates for reducing inductance in semiconductor packages as described above. In addition, the lead frame of the invention locates the ground and power plates on either side of the lead fingers and package leads. This allows stresses on the lead frame to be balanced, and reduces bowing of the package. In addition, improved heat transfer from the die is provided, and an impedance of the package leads can be matched to other electrical components, such as signal leads of a module for the package.